This invention relates to voltage-controlled oscillators (VCOs), and more particularly for VCO with a current-adaptive resistor.
Precise clocks are useful in many electronic systems. Clock generator circuits can be adjustable, such as by using a voltage-controlled oscillator (VCO). The frequency of the generated clock can be adjusted by adjusting the voltage that is input to the VCO. Oftentimes the VCO is part of a phase-locked loop (PLL).
FIG. 1 shows a prior-art PLL. A reference clock and a feedback clock are applied to phase detector 10, which compares the edges of the 2 clocks and generates UP and DOWN signals to charge pump 12. Charge pump 12 responds by charging or discharging a loop filter that includes capacitor 22 and resistor 20. The voltage across the loop filter changes, altering the voltage input to VCO 14. VCO 14 oscillates at a frequency determined by the input voltage from the loop filter.
Clock driver 16 buffers the output clock from VCO 14 to generate the PLL output clock. The generated clock can be divided by divider 18 before being applied to phase detector 10 as the feedback clock.
A VCO is commonly used in PLL control systems, including communications, and timing circuitry. Some applications need a wide VCO frequency range with good linearity and high bandwidth.
One conventional construction for a VCO includes a first portion which converts an input voltage to a control current, and a second portion which converts the control current into an output signal using a ring oscillator. Conventionally the first portion of the VCO is designed to have a linear response in order to effect a linear VCO input/output response. However, this linear control current does not take into account a non-linear current-to-frequency response of the second portion, the ring oscillator.
In order to reduce power line noise, the conventional construction for a VCO includes a capacitance between a ring oscillator current-supply node and GND. VCO bandwidth is limited by this capacitance and the effective resistance of the ring oscillator.
The good linearity and high bandwidth of VCO directly influences the PLL's performance such as reference clock range.
Many different types of circuits can be used as the VCO. A ring oscillator can be used, and its oscillation frequency changed by adjusting the power-supply voltage applied to the oscillator's gates. A VCO using a current-adaptive resistor is desirable.